A Schottky barrier diode (SBD) is widely used in a rectifier circuit for high frequency because the SBD has high-speed switching properties and less forward loss. A conventional SBD has a structure having a cross section shown in FIG. 9, for example. That is, reference numerals in FIG. 9 denote elements as described below. Numeral 1 denotes an n.sup.+ -type semiconductor substrate composed of silicon or the like, for example. Numeral 2 denotes a semiconductor layer which is grown by epitaxial growth on the semiconductor substrate 1 and is an n.sup.- -type active layer, for example. Numeral 3 denotes a metal layer composed of molybdenum (Mo) or the like and forming having a Schottky barrier. Numeral 4 denotes a guard ring having a p.sup.+ -type dopant diffused into a surface of the semiconductor layer 2 near an outer periphery of the metal layer 3 and formed in order to increase a withstand voltage in the periphery of a Schottky junction. Numeral 5 denotes an insulating film which is formed on the surf ac e of the semiconductor layer 2 by thermal oxidation, CVD (chemical vapor deposition) or the like and composed of SiO.sub.2 or the like, for instance.
The properties of a forward voltage V.sub.F and a reverse leakage current I.sub.R of the SBD obtained by the Schottky junction of the metal layer 3 and the semiconductor layer 2 are changed as shown in FIG. 10 in accordance with an inherent barrier value .phi..sub.b of a metallic material and the semiconductor layer. Ti, Mo or the like is practically used as the metallic material for obtaining this type of Schottky junction in view of ease of treatment, economy, reliability and so on The forward voltage and the reverse leakage current are determined in accordance with the barrier value of the material. There is a reciprocal relationship between the forward voltage and the reverse leakage current. Thus, the material having less leakage current has a high forward voltage, whereas the material having a low forward voltage has more reverse leakage current. Both of the leakage current and the forward voltage cannot be therefore reduced.
On the other hand, the structure shown in FIG. 11 for increasing a reverse withstand voltage by reducing the reverse leakage current of a Schottky barrier semiconductor device is disclosed in Japanese Patent Publication No. 59-35183. That is, numerals 1 to 5 in FIG. 11 denote the same elements as the elements having numerals 1 to 5 in FIG. 9. Numeral 6 denotes stripe-like or spot-like p.sup.+ -type semiconductor regions formed on the surface of the n.sup.- -type semiconductor layer 2 that is the active layer. The semiconductor region 6 has the structure for increasing the withstand voltage by reducing the reverse leakage current by a depletion layer formed on the side of the semiconductor layer 2. In this structure, the p.sup.+ -type semiconductor region is not an active region and thus the area of the active region is reduced.
As described above, the properties of the Schottky barrier using the practical metallic material for forming the conventional Schottky barrier have the properties of the forward voltage and the leakage current depending on the material. Thus, the reciprocal properties cannot be avoided. In order to reduce the reverse leakage current, the semiconductor region of a different conductive type from the conductive type of the semiconductor layer (e.g., a p-type region with respect to an n-type semiconductor layer) is formed on the surface of the above-described semiconductor layer that is the active layer. In this case, the p-type region is not the active region and thus the area of the active region of the semiconductor layer is reduced. When the area is reduced, a problem is caused. That is, a serial resistance between the metal layer and electrodes provided on a rear surface of the semiconductor substrate is increased, and consequently the forward voltage is increased. The Schottky barrier semiconductor device is characterized by the low forward voltage. For the recent lightening, thinning and reduction, power saving and low-voltage driving of electronic equipments, the high-performance Schottky barrier semiconductor device is required in which both of the forward voltage and the reverse leakage current are further reduced without increasing a chip area.
Heretofore, one problem has been that the reverse withstand voltage is increased as disclosed in Japanese Patent Publication No. 59-35183, for example. In order to increase the reverse withstand voltage, it is necessary to increase the distance between the lower end of a p-type diffusion region and the lower end of the semiconductor layer 2. This distance is about 3 to 4 times the distance between the p-type diffusion regions 6. Thus, another problem occurs. That is, the forward serial resistance is further increased and thus the forward voltage is increased. On the other hand, in many cases, the Schottky barrier semiconductor device has been recently used together with IC or the like at a low voltage of a secondary power source. Thus, the reverse withstand voltage has only to satisfy tens of volts, e.g., about 30 V, whereas the Schottky barrier semiconductor device having the much lower forward voltage and the still less leakage current is required for the power saving and low-voltage driving of the electronic equipment as described above.